Polarization Exploration #10600

Transcription

1 1550 Finntown Road Waldoboro ME Polarization Exploration #10600 the light wave is always present, but it s omission from the diagrams and figures is solely for clarity of understanding the process. Generally, ordinary light is unpolarized, meaning the orientation of the electric field vector is a random value. Polarizers have the property of selectively allowing light with specific orientations of the electric field vector to be reflected or transmitted, in effect, producing a beam of light with a specific orientation, or polarization.. Figure 1 Polarization materials. Materials: This kit includes: 2 Squares of Material A 2 Squares of Material B 1 Square of Material C 2 Mirrors 1 instructions Polarization: Before discussing the polarization of light, it s useful to talk briefly about light itself. Light is a transverse wave, much like a wave on a taught string. In the case of the string, if we were to make a mark on the string, we d notice that this mark wiggles up and down or sideways, but never moves along the length of the string. The mark on the string is oscillating perpendicular to the direction in which the wave travels along the string. In the case of light, the oscillations don t occur on a string, but rather between electric and magnetic fields. These oscillations are both perpendicular to each other and to the direction of the light wave s propagation. To simplify matters a little bit, he remainder of this discussion will refer to the oscillating electric field of light. The magnetic component of Light (and electro magnetic waves in general) is often polarized by reflection from a surface. This effect is maximized at a specific angle of incidence know as Brewster s angle. Light can also be polarized by selective absorption. This is what occurs in the more commonly available polarizing films used in sunglasses, photographic filters, and the materials used in this kit. These films are produced by blending long molecular chains in a plastic film and then stretching the film in one direction to align these long molecules. When light passes through this film, the rays with electric fields oscillating parallel to the molecular alignment will pass through while those with electric field oscillations perpendicular the molecular alignment in the polarizing film will be absorbed. Linear (Planar) Polarization: Light is said to be linearly or planar polarized if all of the electric field vectors (oscillations of the electric field) of the light are parallel to each other. As an illustration, lets consider the wave traveling on a string again. This time, we ll pass the string through one of the slots in a vertical picket fence. When we shake the string sideways, the wave reaches the fence and then the slats on either side block the wave so that it doesn t pass through. However, if we wiggle the string up and down, in the same direction as the slots in the fence, this time the wave passes through unimpeded. #10600 Polarization Exploration Page 1 of 5

2 Retarding Films: Some materials, though optically transparent, will affect the light passing through them in different ways depending upon the orientation of the electric field vector with respect to the material. In the case of retarder films, the speed of light passing through the material is different for different axes of the film. If the electric field of the light is orientated along one of the film axes, it will pass through faster than if it is aligned with the other axis. This type of material is described as being birefringent. Figure 2: The Picket Fence model of polarization. Circular polarization: Circular polarization may sound like a contradiction in terms, and as such, it s a concept that s a little trickier to visualize. In this case, the electric field vector of the light wave rotates about the direction of propagation in either a clockwise or counter-clockwise fashion. We ll discuss this in more detail after you ve had a chance to explore polarization a little bit further with the materials provided. The Exploration: Keep in mind that there may be a front and back side to each material, so when you perform the following explorations note the orientation of the lettering on the squares of material. Material A - Transmission: Select a square labeled Material A. Note it appearance as light shines through the material. What do you notice about this material? How much light is blocked by the material? Does this change if you flip the material over? Figure 4: Light passing through a polarizing filter. Material A - Reflection: Place the square of Material A over one of the mirrors. What do you see in this instance? What happens if you flip the square over, does the appearance change? Material A, Material A - Transmission: Select both squares labeled Material A, and place one on top of the other. What do you notice about the amount of light passing through both pieces of material as compared to just one piece of material. Try rotating one of the squares in relation to the other. Do you notice any changes? What happens if you flip one of the squares over? Try rotating it again with respect to the other square. How does this compare to the first time you rotated the two films? Try moving the front square to the back, and repeat these experiments. Does it matter which square is in front of the other? Figure 3: The Fast and Slow axis of a retarding film. Material A, Material A - Reflection: Repeat the experiments you just tried for the transmission of #10600 Polarization Exploration Page 2 of 5

3 light through two sheets of material A, except this time place the squares over one of the mirror squares. Do you notice anything different? Material A Conclusions: From the experiments you just performed, what can you say about material A? How does the light intensity vary as one square is rotated with respect to the other? What orientation do the squares have in relation to each other when the most light passes through the material? What orientation do the squares have when the least light passes through the material. Can you create a model that would describe what s happening? Material C - Transmission: Select a square labeled Material C. Note it appearance as light shines through the material. What do you notice about this material? How much light is blocked by the material? Does this change if you flip the material over? Material C - Reflection, one mirror: Place the square of Material C over one of the mirrors. What do you see in this instance? What happens if you flip the square over, does the appearance change? Can you offer an explanation for this change? What happens if you rotate the square of Material C with respect to the mirror? Material C - Reflection, two mirrors: Tape the two mirrors together so that they form a right angle. Look through the square of Material C so you can see your reflection from only one of the mirrors. Does it appear dark? If not, flip it over so that it does appear dark. Now position your viewpoint so that you look into the corner of the two mirrors. Does the appearance of the film change? How can you explain this? Material B - Transmission: Select one square of Material B. Describe what you see as light passes through it. Do you notice any changes when you flip it over? Material B - Reflection: Place the square of Material B on one of the mirrors. What do you see? Are there any changes when you flip it over on the mirror? Material A and Material C - Transmission: Place a square of Material A over Material C so that both labels are readable and on the bottom edges. Look through them at a sheet of white paper. How does the overlapped region compare to the single layers of either sheet? Now move Material A from the front side to the back side of Material C while keeping the same orientation. Do you notice any changes? How does the color of the overlapped region compare? Now rotate the sheet of Material A 90 degrees. Do you notice any changes in the overlapped region? Can you explain these changes? Keeping the same orientation of Material A, move it back to the front side of Material C. Do you notice any changes in the overlapped region? How does this compare to your earlier experiment with two linear polarizers? Material A and Material C - Reflection: Begin with Material C placed on the mirror so that the label is readable and on the bottom right edge of the square. Does it appear dark? Place the sheet of Material A over Material C so that the labels are readable and on the bottom edge. Do you see any changes? Now place Material A behind Material C. Do you see any changes. Rotate Material A 90 degrees. Note any changes. Move Material A from the back to the front of Material C. Do you notice any differences? Now Flip Material C over and place it on the mirror. Did it become clearer? Figure 5: Orientation of mirrors for double reflection. Keeping the labels of both materials with the same orientation, place Material A over Material C. Do you see any changes in color or darkness? #10600 Polarization Exploration Page 3 of 5

4 Move Material A from the front to the back of Material C. Do you notice any changes? Now rotate Material A 90 degrees. What happens to the color of the overlapped region? Keeping the same orientation, move Material A to the front of Material C. Do you see any changes? Material A and Material C - Summary: In this set of experiments, you should have noticed some similarities between A-A interactions and A-C interactions. However, Material C appears to have a front and a back side that affects the experiments. Can you form a hypothesis that might explain this difference? Material A and Material B - Transmission: Select a square of Material A and a square of Material B. Place Material B over Material A so the labels are readable and on the bottom edge. Note the appearance. Move Material B to the back side of Material A. Do you see any changes? Rotate Material B 90 degrees. Does the appearance change? Keeping the same orientation, move Material B to the Front of Material A. Do you see anything different? Material A and Material B - Reflection: Begin by placing a square of Material A over one of the mirror pieces so the label is readable and on the bottom edge. Place a square of Material B over Material A with the same orientation. What do you see? Move Material B between Material A and the mirror. Do you notice anything different? Slowly rotate Material B through 90 degrees. What do you see? When is the overlapped region the darkest? Do you see the same effect when you rotate Material B when it s in front of Material A? Material A, B, and C - Transmission: Make a list of possible combinations of Order and Orientation for materials A, B, and C. Try each possible combination and see what you observe. Some of the more interesting combinations are: Begin with Material A on top of Material C with the labels readable and on the bottom edge. With Material B rotated 45 degrees, slip it between A and B and describe what you see. Next, remove material B and then rotate material A 90 degrees. The overlap should become dark. Now slip material B between A and C and rotate 45 degrees, now what do you find? Can you offer an explanation for what you re seeing? What happens if you place material A behind C and insert B between them? Try rotating material A by 90 degrees - do you see any changes? Material A, B, and C - Reflection: Play around with various combinations of A, B, and C when they re placed on a mirror. How does this compare to what you saw for the previous experiment? Material A - Explaination: Material A is a linear polarizing film. This type of film will allow light of a particular orientation to pass through. A visual model that s often used to describe the polarizer is a rope passing through the slot of picket fence. If you wiggle the rope to create waves, only those waves that are wiggling up and down parallel to the slot will pass through, those waves that are wiggling perpendicular to the slot will be blocked. The action of light as it passes through the polarizing film is somewhat more complicated, but the essence of the description is similar. When two of these picket fences are placed one in front of the other, you can see that the slots of the two fences must be aligned if the waves are to pass through both fences. If one fence is rotated with respect to the other, then the waves that passed through the first fence/filter (called the polarizer) will be blocked by the second fence/filter (called the analyzer ) as they are rotated with respect to one another. It doesn t matter which piece of polarizing film is placed in front or behind the other, nor does it matter if the polarizer is flipped over. The only action that changes the amount of light that will pass through the two filters is the relative rotation of the two filters with respect to each other. Material B - Explaination: Material B is a retarding film. This type of film doesn t reduce the intensity of light passing through it, however it has the interesting effect of allowing light oriented along one direction (the Fast axis) to pass through more quickly than light orientated along the other direction (the Slow axis). If polarized light is incident on the film with the axis of polarization somewhere between the Fast and Slow axes of the retarding film, then the component of the polarized light in the direction of the Slow axis will take slightly longer to get through the film than the component aligned with the #10600 Polarization Exploration Page 4 of 5

5 Fast axis. This delay introduces a phase shift between the two components of the polarized beam. overlapping layers between your polarizing films and see how they behave. Also try stretching different plastic films while looking at them between the filters. Figure 6: The Fast and Slow components of a polarized light beam. Figure 8: How the phase shifted waves produce a rotating electric field vector. Material C - Explaination: Material C is merely a combination of Material A (linear polarizing filter) and Material B (a retarding film). This explains why it appears to have a front and a back side. It also explains why the material appears dark when placed in front of a mirror. In this case, the retarding film is between the polarizer and the mirror. When flipped over so that the retarding film is away from the mirror, the result is that of a linear polarizer alone and it does not appear dark. Figure 7: How a phase shift is introduced by the retarding film. When the components are combined again, this phase shift results in a rotating electric field vector when the light emerges from the film. This type of double ply film is commonly referred to as a Circular Polarizing Film and finds use in many places such as sun glasses, anti reflection screens for computer monitors, and as camera filters. The interesting way that circular polarizers appear dark when looking into a mirror while linear polarizers do not, is because the mirror flips the direction of the electric field vector. For the linearly polarized light, this flipping of the vector doesn t change the orientation and the light is able to bounce back through the same filter. However for circularly polarized light, the flipping of the electric field vector changes the direction of rotation, so what might have originally been a clockwise polarization becomes a counter clockwise polarization and is no unable to pass through the clockwise filter that created the polarization in the first place. Many optically transparent materials exhibit these properties. You may want to experiment with cellophane food wrap, mica, and other clear plastic sheets. Stack up several #10600 Polarization Exploration Page 5 of 5